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Optimization and analysis of 3D nanostructures for power-density enhancement in ultra-thin photovoltaics under oblique illumination

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Abstract

Nanostructures have the potential to significantly increase the output power-density of ultra-thin photovoltaic devices by scattering incident sunlight into resonant guided modes. We applied a modified version of the direct-binary-search algorithm to design such nanostructures in order to maximize the output power-density under oblique-illumination conditions. We show that with appropriate design of nanostructured cladding layers, it is possible for a 10nm-thick organic absorber to produce an average peak power-density of 4mW/cm2 with incident polar angle ranging from −90° to 90° and incident azimuthal angle ranging from −23.5° to 23.5°. Using careful modal and spectral analysis, we further show that an optimal trade-off of absorption at λ~510nm among various angles of incidence is essential to excellent performance under oblique illumination. Finally, we show that the optimized device with no sun tracking can produce on an average 7.23 times more energy per year than that produced by a comparable unpatterned device with an optimal anti-reflection coating.

© 2014 Optical Society of America

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Figures (4)

Fig. 1
Fig. 1 Device geometries considered in this paper. (a) type 1, (b) type 2, (c) type 3, (d) type 4. See text for details. ω, θ, φ represents the angular frequency of incident light, polar angle and azimuthal angle, respectively. PEC = perfect-electric conductor. TE and TM polarizations are defined as shown by the directions of the electric field vectors.
Fig. 2
Fig. 2 Optimized devices for type 1 (a), 2 (b), 3(c) and 4(d). Cross-sections through the bottom scattering layers are shown below each type. Dimensions are in nm and the figures are not to scale. Absorption spectra averaged over all angles of incidence (polar angle from −90° to 90° and azimuthal angle from −23.5° to 23.5°) of the corresponding devices are in (e)-(h). Red and blue dashed lines correspond to TE and TM polarizations, respectively. The black solid lines correspond to unpolarized illumination (average of TE and TM). Absorption spectrum of the reference device with ARC is shown by purple dashed lines. Field-intensity distributions in the active layer under normal incidence for certain absorption resonant peaks are shown in (i)-(l). The field patterns labeled with XZ gives the electric field distribution in the XZ plane at Y = 0. A similar definition applies to the field patterns labeled with YZ. The J-V curves again averaged over all angles of incidence are shown in (m). Comparisons of the peak power-densities, open-circuit voltages and short-circuit current-densities for all devices are shown in (n), (o) and (p), respectively. In each figure, the reference device with ARC is denoted by red lines.
Fig. 3
Fig. 3 Comparison of the optimized device of type 1 (from Fig. 2(a)) against a reference device optimized under normal illumination. (a) Geometry of the reference device that is optimized under normal illumination. Cross-section through the bottom scattering layers is shown. (b) The current-density and peak power-density averaged over all angles of incidence as a function of voltage for the optimized device 1 (red) and reference device (blue). (c) and (d) 2D plots of peak power-density as a function of polar angle and azimuthal angle for the reference device and optimized device 1, respectively. (e) Peak power-density as a function of polar angle. (f) Peak power-density as a function of azimuthal angle. Note that for polar angle analysis in (e), we averaged the peak power-density over all azimuthal angles, while for azimuthal angle analysis in (f), we averaged the peak power-density over all polar angles.
Fig. 4
Fig. 4 (a) Impact of incident polar angle on absorption spectrum of the reference structure and the optimized structure 1. Solar photon flux as a function of wavelength is shown in the inset. The absorption spectra are taken at an azimuthal angle of 0°. (b) Field patterns for the reference device and optimized device 1 under the polar angle of 0°, 30°, and 60°, respectively. The field patterns in “XZ” column are taken in the XZ plane at Y equals zero, while the field patterns in “YZ” column are taken in the YZ plane at X equals zero. All the field patterns are calculated at zero azimuthal angle.

Tables (1)

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Table 1 Ranges and unit perturbations of geometric parameters for optimization

Equations (6)

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j ( V ) = q ( F s F c o exp ( q V k T c ) ) .
F s = ω g d ω S ( ω ) A ( ω , θ = 0 , φ = 0 ) ,
F c o = 0 2 π d ϕ 0 π / 2 d θ ω g d ω Θ ( ω ) A ( ω , θ , φ ) cos ( θ ) sin ( θ ) ,
P(V)=j(V)V.
V oc = k T c q log( F s F co ).
J sc =q( F s F co ).
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